The present invention relates to a phase change optical disk suited to carry out overwrite using a single beam (referred to as "single beam overwrite") and a method for using the optical disk. More particularly, the present invention relates to a phase change optical disk which can provide a uniform reproduction output characteristic at positions on inner and outer sides thereof, and a recording for erasing method therefor.
Generally, the phase change optical disk utilizes a change in the optical property of a recording film used, which occurs owing to the phase state change thereof, to record and erase information; the phase state of the recording film in the optical disk corresponds to the process of heating and subsequent cooling (radiation) of the recording (or erasing) position of the recording film by laser beam irradiation. On the other hand, it has been demanded in the technical field of the phase change optical disk that the single beam overwrite technique, which can erase recorded information and also record new information by only irradiating the optical disk with a single laser beam with modulated power (i.e. modulated amplitude in light intensity), can be put into practice. The single beam overwrite in the phase change optical disk is disclosed in for example, the technical report in DENSHI JOHO TSUSIN GAKKAI, (The Institute of Electronics, Information and Communication Engineers of Japan) CMP 87-88, 89, 90 (1987).
A vertical structure of such a conventional optical disk 58 is shown in FIG. 1. In FIG. 1, 51 is a disk-shape PC (polycarbonate) substrate. Formed on the substrate 51 is a recording film 53 made of antimony (Sb), selenium (Se) and bismuth (Bi) which is 120 nm thick. Further formed on the recording film 53 is a protection film 56 made of an ultraviolet ray setting resin. Incidentally, 30 is a hole centrally provided on the principal surface of the optical disk 58.
In carrying out the single beam overwrite, the phase change optical disk 58 is irradiated with a laser beam having a modulated irradiation power P as shown in FIG. 2. In FIG. 2, the level of recording information is at a high power Pw, and the level of erasing information is at an erasing power Pe which is lower than Pw. Such modulated irradiation power erases the previous information recorded on the optical disk and also records new information. The level of reproducing information which is at the lowest power Po is used to reproduce the recorded (or erased) information to provide a reproduced signal.
In case of the prior art, when the number of rotations of the optical disk 58 is fixed, and the single beam overwrite is carried out, the ratio of a carrier level to random noise (referred to as "CNR") during the irradiation of the laser beam with the same power is lower at the outer position of the disk where the linear velocity on the recording surface (principal surface) of the optical disk 58 is higher. The linear velocity v is defined as EQU v=2.pi.rN/60
where r indicates the radius of disk and N indicates the rotation number (rpm). On the other hand, cross-talk is less at the outer position. This is because the irradiation density of the irradiation power P of the laser beam is lower at the outer position, more specifically because the width over which information is recorded on a certain track (not shown}of the optical disk 58 is narrower at the outer position. Therefore, in order to provide the output characteristic of a substantially uniform reproduced signal at both inner and outer positions, a larger irradiation power P is required at the outer position. However, since an available output level from a semiconductor device (laser diode) has a limit, sufficient irradiation power P could not be obtained.
Additionally, if the number of rotations (rotation speed) of the optical disk 58 is suitably set so as to fix the linear velocity thereof, the above problem does not occur. In this case, however, a motor used is required to vary the number of rotations in accordance with the radial position of the optical disk where an optical head is positioned. This makes it difficult to carry out high speed random access which is a great feature of the optical disk in its use, so that the above fixing of the linear velocity can not be actually adopted.
Furthermore, there has been proposed the related art as disclosed in U.S. patent application Ser. No. 362699 filed on Jun. 7, 1989 and U.S. patent application Ser. No. 366,873 filed in Jun. of 1989 both assigned to the present assignee. The single beam overwrite in this related art will be explained briefly. FIG. 3 shows a vertical arrangement of the optical disk on the related art. In an optical disk 68 shown in FIG. 3, formed on a glass substrate 61 is a recording film 63 made of indium (In), antimony (Sb) and tellurium (Te), which is about 30 nm. Formed on the recording film 63 is an interference film 64 of Si.sub.3 N.sub.4 which is about 70 nm thick. Formed on the interference film 64 is a reflective film 65 of gold (Au). Finally formed on the reflective film 65 is a protection film 66 of Si.sub.3 N.sub.4. A numeral 30 is a hole centrally provided on the principal surface of the optical disk. The optical disk 68 is irradiated with a laser beam 9 from its bottom surface.
The characteristic of the optical disk in the case where the disk is irradiated with the laser beam converged in about 1 .mu.m while it is rotated will be explained.
FIG. 4 shows the relation between the temperature T of the recording film reached when the recording film 63 at a recording position (or erasing position) is heated at temperatures above its melting point and the cooling rate Cs of the recording film when it passes a crystallization temperature zone, over which the crystallization of the recording film 63 is promoted, in the cooling process after having been melted; the cooling rate is measured in terms of the temperature lowered during unit time. As indicated by a solid line, the cooling rate Cs of the recording film 63 increases as its temperature rises.
if the recording film 63, after melted, is cooled at a sufficiently high cooling rate Cs, it will be made amorphous; otherwise it will be made crystalline. The minimum cooling rate Cs required for the recording film to become amorphous is the critical cooling rate thereof.
Now it is assumed that the critical cooling rate of the recording film has been set at a value Csc indicated by a broken line in FIG. 4. (The critical cooling rate can be set optionally set by changing the components and/or composition of the recording film). If the recording film 63 is heated to temperatures above the critical temperature Tc decided a point of intersection of the solid line and broken line, the cooling rate of Cs will exceed the critical cooling rate so that the recording film 63 at a recording position, after melted, will be made amorphous, i.e. placed in the recording state.
On the other hand, if the recording film 63 is only melted to a temperature T in the range from the melting point Tm to the critical temperature Tc, the cooling rate Cs will become lower than the critical cooling rate Csc so that the recording film 63 at the recording position, after melted, will be crystalline, i.e. placed in the erasing state. It should be noted that even if the temperature of the recording film does not reach the melting point Tm, the crystallization thereof will proceed as long as the temperature is higher than a glass transformation temperature Tg.
The temperature T of the recording film 63 can be controlled by the irradiation power P of the laser beam 9 so that the recording and erasing can be easily realized by only controlling the irradiation power P. Thus, the single beam overwrite can be also realized.
The above recording/erasing method (generally referred to as "melt-erasing method") once melts the recording film regardless of recording and erasing so that it can reduce incomplete erasure of information as compared with the conventional technique of erasure by crystallization from the solid phase, thereby providing high erasability and a high CNR.
Thus, the basic principle of one example of the single beam overwrite has been explained in connection with the relation between the temperature T of a recording film and the cooling rate Cs thereof.